In the course of the past decade, the development and use of expandable stents in the medical field has grown enormously, to the point where stents today form a common element in the treatment of a wide variety of complaints and illnesses. Stents typically have a generally hollow cylindrical form and comprise a relatively intricate structure or framework of interconnected struts or arm members, which can be cut from a plain cylindrical blank. The cylindrical blank may be comprised of e.g. stainless steel, cobalt-chromium alloy, or nickel-titanium alloy (Nitinol).
Given the relatively fine and intricate nature of the stent structure, the stent framework is very often produced by laser cutting the cylindrical blank. While the laser cutting procedure is highly desirable in its ability to produce very small and yet very precise stent structures, it nevertheless has the disadvantage that it also generates residues of melted and re-solidified material (known as “slag”) along the edges of the cut material. These slag residues produce rough or uneven deposits on the surfaces or edges of the stent struts or arm members, particularly on the inner surfaces and edges of the stent structure. The slag deposits and residues from the cutting procedure may normally be largely removed with a light scouring, scraping or abrading procedure. Nevertheless, even after such scraping, some remnants of the deposits remain, leaving burrs, cracks, pitting and/or surface unevenness.
Furthermore, the cylindrical metal blanks (e.g. stainless steel, cobalt-chromium alloy, or Nitinol) which are used to produce the stents are typically manufactured in a tube drawing process. This drawing process also typically generates roughening of the tube surfaces. The manufacturer of the tubular blanks will typically machine the outer surface of the tube material, e.g. by grinding, to a high level of smoothness before supplying the blanks for cutting into stents. The inner surfaces of the blanks, however, which later form the inner surfaces of the stent struts or arm members, are not ground prior to supply for stent production and therefore retain their roughness from the drawing procedure.
Importantly, it will be appreciated that burrs, pits, cracks, and other such surface imperfections can lead to stress concentrations and create weaknesses in the framework of the stent structure, thereby increasing the potential for failure of the stent. That is, such surface imperfections can lead to a localised overloading of the stent framework resulting in fracture, especially where multiple expansions of the stent occur, as is the case with Nitinol stents. Furthermore, pits, cracks and other such surface imperfections also serve as sites for the onset of corrosion, which may weaken the stent and potentially lead to failure of the stent in situ, i.e. in the patient. Accordingly, the removal of burrs, pits, cracks, and other such surface imperfections to provide very smooth stent surfaces is highly desirable for ensuring both fatigue resistance and corrosion resistance in use.
Although the insertion of a sleeve inside the cylindrical blank during the laser cutting procedure may be helpful to reduce splatter and slag formation on the stent, this technique does not completely eliminate slag formation and also has no bearing on the surface roughness created during formation of the blank by tube-drawing.
Thus, the use of an inner sleeve during laser cutting makes the cutting procedure more complicated and ultimately does not solve the problems of surface roughness.
A number of techniques have been employed to remove surface roughness from stents. One technique employed to smoothen stent surfaces is electro-polishing. Such a polishing technique, however, often proves ineffective for removal of excessive surface roughness because of the high rate of mass removal necessary. Chemical etching has also been employed to address the surface roughness and slag formation prior to electro-polishing. The chemical etching procedure, however, has the disadvantage that it removes material from all surfaces of the stent, even when the surface roughness to be treated is concentrated at the inner surfaces (e.g. on the internal diameter) of the stent structure. Furthermore, because pits and cracks in a surface are etched along with burrs and projections, chemical etching will not readily produce a plane surface. In addition, the chemical etching compound may not penetrate especially narrow or tight cracks and fissures, thereby potentially leaving regions of weakness in the stent surface. Thus, the use of chemical etching to smoothen the internal surfaces of the stent will generally remove more material than is necessary (e.g. from the external surfaces and side walls of the stent), will generally not produce a planar surface, and may not address all surface cracks and fissures.
Another technique for smoothing the surface roughness involves machining the inner surfaces of the stent to remove any remnants of slag and/or burrs and to eliminate pitting and cracks. In a conventional machining procedure the stent is held between parallel clamping elements or jaws and a file is inserted into the stent. This technique has the disadvantage, however, that the clamping elements or jaws tend to deform the stent and cause an uneven removal of material from the internal stent surfaces. Furthermore, the pressure from the clamping elements or jaws can cause the struts or arms of the stent framework to twist or bend during rotation of the file.